Polymer materials are ubiquitous in everyday life and are used in various applications (medical, automobile, electronics, structural etc.). These materials experience stress through normal use, which can lead to damage and failure of the product. Having the ability to detect damage and locate areas under high stress in situ is essential to eliminating failure of the material.
Several examples of self-assessing materials are known in the patent literature. The simplest incorporate a colored substance into the matrix in the form of capsules1 or hollow fibers.2 Initially the color is not visible, but, upon damage to the matrix, the capsules or fibers rupture and expose the colored fluid or solid. In some cases, a two part system is utilized wherein a colorless compound mixes with an activator upon the rupture of their respective containers causing a color change. The disadvantage of these systems is that the fibers and capsules need to be evenly dispersed throughout the matrix, so that the damage inducing force has a large chance of intersecting the particles.
Another approach is the use of triboluminescent materials, which give off flashes of light in response to stress or damage.3 These materials require continuous monitoring to detect when damage occurs due to the transient nature of the light flash.
Smart coatings consisting of several layers of sensing materials have also been reported.4 These are complex and require external power to accomplish many of their tasks. A diacetylene segmented copolymer is known which exhibits a shift in color when subjected to a strain.5 
In the chemistry literature, Todres outlines several organic compounds that have displayed mechanochromic properties.6 Specifically, spiropyran has been noted to undergo a color change upon grinding;7 however, little application exists for the small molecule alone.
Weder and coworkers have incorporated cyano-substituted oligo(p-phenylene vinylene) derivatives into different polymer matrixes and have synthesized “self-assessing” polyurethanes, polyethylene blends, poly(ethylene terephthalate)s, and poly(ethylene terephthalate glycol)s.8a-e Their approach relies on the initial formation of nanoscale aggregates of the sensor molecules in the polymer matrix. Upon deformation the cyano-substituted oligo(p-phenylene vinylene) sensors are transformed from excimer to monomer and a shift in the emission spectrum is observed. Most of these sensing units are not chemically incorporated into the backbone, and many exhibit only a fluorescent color change that is not visible to the naked eye. Additionally, these materials are not reversible and can only exhibit a color change once.
Finally, Kim and Reneker introduced an azobenzene into a copolyamide oligomer, which was chemically incorporated into a polyurethane.9 Upon exposing the material to tensile stress, a change in the UV spectrum at 375 nm was observed. However, no visible change was noted and the polymer had to be irradiated with UV light prior to stressing the material.